Many modern communication systems transmit messages using signals having amplitudes that vary significantly over time. Typically, the power capability of the output device in the transmitter (e.g. the final amplifier) must be selected to accommodate the peak amplitude, or perhaps the amplitude at the 99th percentile, below which 99% of the amplitudes in the signal occur, or perhaps at the 99.9th percentile as another example. In other words, the selection of the output device is governed as much (or more so) by the peak-power requirement as it is by the average power requirement, where the average power is typically the measurement specified directly in the specification document for a communication system. The selection of the output device is governed as much (or more so) by the peak-amplitude requirement as it is by the RMS-amplitude requirement. Therefore, the output device selection requires tradeoffs between accurate reproduction of the signal on the one hand (hereinafter referred to as “communication quality”) and, on the other hand, power efficiency, power consumption, size of the output device, etc. (hereinafter referred to as “device efficiency”).
Improving the output device efficiency, especially the power efficiency, is highly desirable since transmitters (such as those found in battery operated cellular telephone) have a limited battery source to operate the device. Additionally, improving the device efficiency is beneficial for transmitters constrained by their cooling means, so that thermal damage to the device (e.g., handheld transmitters operating with high duty cycle, or high-power transmitters in base stations in cellular telephone systems) can be prevented. For these applications and others, it is desirable to minimize the ratio between the peak amplitude (or 99th percentile or 99.9th percentile) and the root-mean-square (RMS) amplitude, in order to facilitate efficient power transmission.
One conventional approach to power-efficient transmission is a so-called linear amplifier that operates in a partially linear and partially nonlinear mode, characterized by a phenomenon known as amplitude compression. In this mode, a small input amplitude A gives rise to a larger output amplitudeB=gAwhere g is the small-signal gain of the amplifier. As the input amplitude A increases, the actual gain of the amplifier decreases below g. Thus, the corresponding output amplitude B is not quite as large as it should be. In other words, the amplifier output is not truly proportional to the input. This effectively reduces the peak-to-RMS ratio, which in turn has the benefits of power-efficiency previously described.
A major disadvantage of the linear amplifier approach is that it degrades the signal quality. Typically, some amount of signal quality degradation is accepted in a system design, in exchange for slightly improved power efficiency or reduced heat dissipation. Nonetheless, the conventional (linear amplifier) approach degrades both standard measures of quality, namely an in-band quality measure and an out-of-band quality measure. The in-band quality measure is the RMS error vector magnitude (EVM). A mathematically related measure is RHO which is the normalized cross-correlation coefficient between the transmitted signal and its ideal version. The EVM and RHO relate to the ease with which an intended receiver can extract the message from the transmitted signal. As the EVM increases above zero, or RHO decreases below one, the transmitted signal becomes increasingly distorted compared to the ideal signal. This distortion increases the likelihood that the receiver will make errors while extracting the message.
The out-of-band quality measure is the power spectral density (PSD) of the transmitted signal, or some measure derived therefrom. Of particular interest in the PSD is the degree to which the transmitted signal interferes with other radio channels. In a wireless communications network, to minimize interference with other radio channels, the overall capacity of the network is reduced or limited (e.g., the number of simultaneous users is reduced or limited).
Therefore, any means of reducing the peak-to-RMS ratio must create as little interference as possible (minimal degradation to out-of-band signal quality) while simultaneously maintaining the in-band measure of signal quality (i.e., EVM or RHO) at an acceptable level. The conventional (linear amplifier) approach degrades both out-of-band signal quality and in-band signal quality to reduce peak-to-RMS ratio. In fact, for some signals the conventional (linear amplifier) approach is unable to maintain acceptable quality while delivering the required average power.
In view of the foregoing, it would be desirable to have a PAM signal generator that reduces the peak-to-RMS amplitude ratio of a communications signal to facilitate efficient power transmission and delivery of required average power, while preserving out-of-band signal quality.
It would also be desirable to have a PAM signal generator that modifies pulse amplitude modulated signals to reduce the peak-to-RMS amplitude ratio of the signals without degrading the power spectral density (PSD) of the signals and while simultaneously maintaining the in-band measure of signal quality (i.e., EVM or RHO) at acceptable levels.